Based on the hypothesis that the genetic structure of the major histocompatibility complex (MHC) in chickens (B locus) parallels the H-2 complex in mice, objective 1 is to identify and characterize immune response (Ir) genes linked to the B complex. Objective 2 is to determine whether the B blood group system in chickens agrees with the 2-locus murine model and the HL-A complex in man. We believe we have available the necessary genetic material to approach objective 1 in 2-3 years and evidence bearing on objective 2 in possibly 3-4 years. The key to our approach is the unique B1B1 genotype segregating in our S1 line which has consistently shown high adult mortality (4 - to 5-fold higher than controls) and has demonstrated immune hyporesponsiveness to Salmonella pullorum bacterin and to equine ferritin. We think this is under genetic control by an Ir gene linked to B1. The base population we have is line S1 (non-inbred) segregating for 3 B alleles: B1, B2, and B19. Also we have 7 inbred lines ranging up to 97% in coefficient of inbreeding. By introducing B1 into each inbred line followed by 3 or 4 backcrosses, we should produce nearly congenic or quasi-congenic lines segregating within the B locus complex. The development of congenic lines allows us the opportunity to determine how well the 2-locus model for MHC with linked Ir genes in mice fits the pattern for MHC in chickens. The fact that the avian species has a clearly defined and easily separable dual immune system (B and T lymphocytes) argues for greater use of chickens in the study of immunobiology.